The third-order nonlinear optical susceptibility has been investigated for oriented thin films of halogen bridged platinum complexes. The | χ(3)(−3ω;ω,ω,ω) | spectrum associated with the third harmonic generation (THG) was measured by the Maker fringe method in the region of fundamental wavelength 0.6 μm∼1μm and 1.6μm∼2.2μ m. In the | χ | spectra, we found a peak structure at ω ∼ 1.8eV that is attributable to a two-photon resonance to the optically forbidden charge transfer exciton. The | χ(3)| value was in the order of 10−11esu that is comparable to the π-conjugated polymers. Common features of NL0 processes for the one-dimensional semiconductors are discussed.